This invention relates to an implantable cardioverting/defibrillating pacemaker and, more particularly, to a pacemaker of this type that is responsive both to an electrically derived heart rate and to an haemodynamic parameter.
An implantable haemodynamic cardioverting/defibrillating pacemaker is disclosed in U.S. patent application, No. 481,364 to K. A. Collins, filed Feb. 16, 1990, and entitled "An Implantable Automatic and Haemodynamically Responsive Cardioverting/Defibrillating Pacemaker," now U.S. Pat. No. 5,083,563 which is assigned to the assignee of the present invention, and is incorporated herein by reference. The pacemaker of said application No. 481,364 overcomes many of the problems of false or inappropriate delivery of therapy that arise out of a reliance solely upon the sensing of the electrical activity of the heart as a means of determining the state of cardiac function.
Among the significant problems that face such an implantable device are to control the quality of its input signals, and to determine when there has been a failure of the input transducing system. These involve three components: first, deciding when a sensing system has failed to properly function; second, properly controlling the gain of the input amplifiers; and third, resolving conflicting information when more than one method of transduction is being used.
The failure of a lead which interconnects the sensing element and the pacemaker, due to breakage of the conductor, is possible. More commonly, failures in sensing are a result of inappropriate input amplifier gain. It is well known that the signal amplitude of the electrocardiogram (ECG) signal varies, especially during ventricular fibrillation (VF), and that an inability to sense the ECG can result in a failure of the defibrillator to revert VF, despite mechanisms for altering the gain of the amplifiers to suit the signal. This is taught by Williams et al. in their article "Automatic Implantable Cardioverter-Defibrillator-Related Complications", JACC, Vol. 15, Page 55A (1990).
A related sensing error can occur due to transient increases in the noise injected into the ECG sensing system. Such bursts of noise can be induced by electromagnetic sources such as electric motors They can be responsible for temporary periods of uncertainty in the diagnosis of the state of the hearts function when it is assessed by electrically derived rate criteria.
For cardioverting/defibrillating pacemakers that use more than one sensing modality, there is a need both to have systems to detect the various forms of sensing system failure and to have methods for initiating appropriate therapy in the face of such failure.
Mirowski et al., in U.S. Pat. Nos. 3,614,955 and 3,942,536, describe systems that sense heart function using the peak of the right ventricular pressure waveform. Devices of this kind, have no means of determining whether or not their sensing circuits are functioning.
A device disclosed in U.S. Pat. No. 4,774,950 to Cohen is an haemodynamic cardioverting/defibrillator. This device also lacks any mechanism by which to monitor its own function. It also lacks any means or method for deciding upon device function in the event that there is either a failure of or a temporary uncertainty in the diagnosis of the state of the heart in one of its sensing modalities.
As indicated earlier, two problems associated with implantable cardioverting/defibrillating pacemaker devices are to control the quality of its input signals and to determine when there has been a failure of the input transducing system. These involve three components, firstly, deciding when a sensing system has failed to properly function; secondly, properly controlling the gain of the input amplifier; and thirdly, being able to resolve conflicting information when more than one method of transduction is being used.
The first component is the easiest to resolve. It is common practice to measure the impedance of pacing electrodes to determine if they are properly sited. Dutcher et al., in U.S. Pat. No. 4,140,131, discloses one method of implementing such an impedance check in an implantable pacemaker. This method does not progress far enough for a device that has more than one means of sensing heart function. The Dutcher et al. method can only determine whether or not the impedance of the pacing electrode is within operational limits. It cannot determine whether or not pacing is still possible; nor can it switch to an alternate sensing mode in the event of failure of the primary sensing modality; nor can it be used to adjust the amplifier gain of the input amplifier.
In European Patent Application No. 89305623, dated Jun. 5, 1989, Baker et al. reveal a method and means of implementing automatic gain control that is based on the amplitude of the ECG signal that is being presented. This system suffers from a basic problem that is common to all systems that use the sensed signal as the means for establishing the gain on the sensing amplifiers, namely that the detected signal can be so small as to be lost in noise. Any amplifier gain applied to the desired signal is also applied to the noise present in the sensing system. Increasing the gain of the sensing amplifiers in such situations, despite anti-noise filtering, will lead to the inappropriate sensing of noise spikes as if they were QRS complexes in the ECG signal.
It is, therefore, a primary object of the present invention to provide a cardioverting/defibrillating pacemaker device which includes a second sensing modality and which is capable of determining the appropriate action to take in the event of a failure of or uncertainty in the diagnosis of cardiac state in one of its two or more sensing systems.
An additional object of the invention is to provide a two sensing modality cardioverting/defibrillating pacemaker device which utilizes electrically derived timing events to diagnose heart function in the event of pressure signal failure and which, in the event of failure of the ECG sensing system, uses a ventricular pressure (VP) signal to determine the state of function of the heart.
Another object of the invention is to provide a cardioverting/defibrillating pacemaker device which can determine whether a malfunction is in the sensing circuit and/or in the pacing circuitry by reference to the evoked response seen in the ventricular pressure (VP) waveform, and to use this information to alter its own therapy logic to best be able to deal with the detected malfunction.
A further object of the invention is to provide a cardioverting/defibrillating pacemaker device to implement a form of gain control that does not rely upon the signal being sensed to determine the state of heart function, while it ensures that the gain of the sensing amplifiers is optimal.
A still further object of the invention is to provide, in a cardioverting/defibrillating pacemaker device that utilizes two sensing modalities, the ability for the pacemaker to use one sensing modality to monitor the function of the heart while the gain of the other modality is being altered. Amplifier gain can thus be controlled and one of the two systems deactivated when an adequate signal cannot be obtained despite maximal amplifier gain.
Yet another object of the invention is to provide a cardioverting/defibrillating pacemaker device having two sensing modalities therein that will provide a tactile warning in the event of one of the two sensing modalities being deactivated, and will inform the supporting physician of such deactivation via telemetry on the next occasion upon which he interrogates the device.
Further objects and advantages of this invention will become apparent as the following description proceeds.